As integrated circuit devices increase in device density and the size of wafers used in integrated circuit manufacturing increase in size, there is a continued need for more precise manufacturing process control.
In wet processes (e.g., wet etching) used in some stages of integrated circuit manufacture, process endpoint control has been achieved by monitoring one or more electrical properties (e.g., impedance) in the liquid bath medium in which the wet process is conducted. The change of such electrical property (ies) is correlated with the actual state of the wafer(s) being processed such that the process can be terminated or altered when a target electrical property value and/or rate of change is achieved. Examples of such processes are described in U.S. Pat. Nos. 5,338,390; 5,445,705; 5,456,788; 5,501,766; 5,516,399; and 5,788,801, the disclosures of which are incorporated herein by reference.
While the known techniques provide some process control, there can be difficulty in interpretation of the electrical signal corresponding to the monitored electrical property. For example, noise or scatter may be present in the signal such that the actual value of the electrical property and/or the rate of change (slope) are difficult to determine accurately. This problem can lead to operator and/or machine error in interpreting when to stop the wet process of interest such that the ideal manufacturing result is not achieved.
Thus, there is a need for improved monitoring techniques and apparatus to provide better electrical property monitoring to enable consistent selection of the desired process endpoint.